Fig. 158.—Biplane Driven by Compressed Air
The upper and lower longerons are spanned at the tail end with light spruce cross-bars, ⅛-in. by ¼-in. section, which are let into mortises cut in the longerons; and two vertical posts are halved on to these cross members ([see Fig. 152]), to provide the fulcrum about which the tail swings in the quadrant, to be referred to presently. They are spaced 4 in. apart, which is equivalent to the distance between two ribs; and on the outside of them a groove is cut in the centre of each to provide a seating for the two central tail ribs. These grooves must be cut V-shaped, the apex of the V facing the trailing edge of the post. The object of the groove is to form a guide for the tail when it is desired to alter the angle of it. A pin should be driven through the rib and into the groove to constitute the pivot on which the tail swings; and the ribs must be bound with fine thread on each side of the pin to prevent the rib from splitting. It will be found that it is better to bind the ribs before inserting the pin.
The central inter-struts are attached to the skids by angle-plates, and in [Fig. 153] the form of these is given. It must be understood that there are two plates to each joint, one on each side of it, and for neatness and simplicity they can be cut from one piece of tin, both plates being thus formed in the one. No. 30 gauge tinplate is suitable. The plates are pinned, clinched, and bound into place, and constitute an exceedingly rigid piece of construction, which is needed in this portion of the machine, bearing as it does the impact of landing. Glue should be neatly brushed into all the joints. The tie-strut is to be streamlined as far as practicable without materially impairing the strength of it.
A very neat finish can be given to the binding if it is just brushed round with japan black, which shows up in pleasing contrast to the light brown varnish with which the framework is coated.
[Fig. 154] gives the shape of the quadrant, which makes possible the variation to the angle on the tail. It is cut to a radius of 4 in., and is pinned into position. The pitch of the teeth is ⅛ in., and this facilitates a very fine adjustment. It should be so fixed that the tail springs tightly into notches, but not so tightly as to render adjustment difficult. Trial and error will be found the best method of locating its position.
Fig. 159.—Compressed-air-driven Monoplane
It was mentioned in the preceding chapter ([see pp. 94 to 103]) that the axle is composed of two portions, umbrella-ribbing and piano wire, and [Fig. 155] shows the construction. It will be seen that the piano wire beds into the channel (which is fixed in a trailing position), wherein it is bound and soldered. The wheels are spaced apart by means of small brass-tube collars, soldered to the piano-wire axles in their respective positions. The axle itself, as mentioned earlier in the chapter, is attached between the shock-absorber brackets, being held there by means of suitable radius wires secured to any convenient part, the rubber binding forming the absorber. The radius wires are essential in order to maintain the lateral position of the axle relatively to the planes. Sufficient rubber binding is to be used to absorb the shocks the model is bound to receive, the exact quantity, of course, being impossible to define.
The rear wheel members are fixed to the longerons in the following manner. The ends are bent back parallel to align with the frame member. The apices of the V chassis members are soldered to the short axle carrying the back wheel, the axle being cut a length suitable to the hub of the wheel. No. 20 gauge wire is used for all portions of the rear chassis. [Fig. 156] makes this clear.